Routers with electronic switch fabrics have the ability to store data packets and then make multiple copies of those packets. In one approach, for example, a packet is copied into an electronic fabric and each output card reads the same copy of that packet. When all of the outputs have read the packet, it can be discarded. Thus the fabric itself is capable of performing a multicast operation, in which one input packet is cloned and copied to multiple outputs.
A router switch fabric that has no inherent storage or replication capability, for example an optical switch fabric, cannot fundamentally multicast packets. There are other approaches that, for example, store a packet on an input line card in electronic form and have the input line card make multiple copies of the packet, sending them to each multicast output as necessary. Although this will function and perform the multicast operation, a drawback is that the packet sits in memory on the input card, and if sufficient numbers of multicast packets arrive, the memory on the input card can be exhausted, or the memory can back up subsequent nonmulticast packets and delay or block them from getting through the switching fabric.